Planar oxygen sensors are used in a variety of applications that require qualitative and quantitative analysis of gases, such as applications that involve automobiles. In automotive applications, the relationship between the oxygen concentration in exhaust gases and the air-to-fuel ratio of a fuel mixture supplied to an engine allows the oxygen sensor to provide oxygen concentration measurements for determining optimum combustion conditions, maximization of fuel economy and efficient management of exhaust emissions.
Generally, a planar oxygen sensor consists of multi-layer ceramic structures and usually has a pump cell, a reference cell, a chamber between the pump cell and the reference cell, a heater and a ground plane electrode between the heater and the reference cell. The function of the oxygen sensor is to pump any residual oxygen from the chamber through the pump cell to the outside environment. As the oxygen ions, which are proportional to the oxygen concentration of the environmental gas being measured (in this case exhaust), flow through the pump cell an electric current proportional to the oxygen ion flow is generated thus allowing the oxygen concentration to be determined.
In many cases, in order to maintain an optimal level of precision and performance, the oxygen sensor should be operated at a fixed high temperature. Usually, to maintain the fixed temperature for best sensor function, temperature control is part of the sensor system with a temperature sensing feedback. As such, in chemical sensors constructed of electrolyte materials, such as zirconia, a typical temperature control incorporates the impedance measurement of the electrolyte material as the temperature feedback.
However, these designs have a number of drawbacks. First, the impedance of the electrolyte material can drift over the service life of the sensor and can affect the performance of the sensor or result in the overheating of the sensor. Second, due to impedance non-uniformities of the electrolyte material, the accuracy of the sensor temperature measurements may vary. Lastly, measuring the impedance of the electrolyte material can be somewhat complicated requiring higher cost sensor control circuitry.